1. Field of the Invention
The present invention relates to improvement in material for tools, made of sintered body (CBN sintered body, hereinafter) consisting mainly of cubic boron nitride (CBN), and in particular to a super hard composite material improved in wear-resistance.
2. Description of the Related Arts
Cubic boron nitride (CBN) is the hardest material next to diamond and is used in cutting tools. CBN sintered body is produced by sintering crystalline particles of CBN with binder (sintering aid). The CBN sintered bodies can be classified mainly into following three types:
(1) CBN sintered body containing 20 to 70% by volume of CBN crystalline particles, binder being carbide, nitride or carbonitride of Ti (i. e. Japanese patent publication-A-53-77811). PA1 (2) CBN sintered body containing more than 80% by volume of CBN crystalline particles, binder being Al and Co metals (i. e. Japanese patent publication-B-52-43846). PA1 (3) CBN sintered body containing more than 95% by volume of CBN crystalline particles, binder being M.sub.3 B.sub.2 N.sub.4 in which M is alkali earth metal (i. e Japanese patent publication-A-59-57967). PA1 (a) nitride or carbonitride of at least one element selected from a group comprising IVb group elements, Vb group elements, IVb group elements, Al and B and possessing a crystal structure of cubic system and metallic bond property (metallic compound, hereinafter), and PA1 (b) at least one compound possessing a crystal structure other than cubic system and covalent bond property (covalent compound, hereinafter), under equilibrium condition at ambient temperature and pressure.
CBN sintered body possesses very high hardness and thermal conductivity (or improved strength at high temperatures) and is used in cutting tools for a variety of steels. For example, the type (1) of CBN sintered body possesses Vickers hardness of 2,800 to 3,700 and shows improved wear-resistance and hence is used in cutting of hardened steels. CBN sintered body of the type (2) possesses Vickers hardness of 3,500 to 4,300 and shows improved resistances to wear and chipping and hence is used in cutting of high-hard cast iron or the like. CBN sintered body of the type (3) possesses Vickers hardness of 4,000 to 4,800 and high thermal conductivity and hence is used in bonding tools or the like.
These CBN sintered bodies, however, have demerits of cleavage and low resistance to acids, so that wear is inevitable in tools made of CBN sintered body alone due to insufficient resistance to wear when cutting work is effected on such materials that are difficult to be cut such as steel for transmission.
It is proposed to coating a surface of the CBN sintered body with a variety of wear-resisting layers so as to improve the resistance to wear of CBN (i. e. Japanese patent publication-A-59-134603, A-61-183187, A-1-96083, and A-1-96084). However, the hardness of known wear-resisting layers are lower than that of CBN sintered body and hence wear is investable.
It is also proposed to coating a surface of cemented carbide tool with TiN, (TiAl)N, TiCN, Al.sub.2 O.sub.3 or the like. In this tool, however, the base of cemented carbide deforms plastically at high cutting temperature, resulting in that the wear-resisting layer peels off or destroyed.
An object of the present invention is to provide a super hard composite material for tools, improved in strength of base material and in wear-resistance and possessing improved hardness and resistance to oxidation at high temperatures, which can be used in cutting work of hardened steels, high-grade high-hard cast iron or the like.